The invention relates to a center disk wheel of the type produced from sheet metal for mounting to the wheel hub of a motor vehicle. More particularly, it relates to a coated center disk wheel and ways to ensure secure mounting of such a wheel.
A center disk wheel is disclosed, for example, in DE 196 03 968 A1. There, it is explained that, during mounting of the wheel on the vehicle, the wheel nuts or wheel bolts are tightened with a predetermined torque, resulting in a pretensioning force in the screw bolts and a correspondingly high pressure between the connecting eyes of the wheel and the wheel hub.
When a wheel nut or a wheel bolt is turned, the moment of friction in the thread, which increases with the pretensioning force, and the moment of friction at the contact surfaces between the nut or the bolt head and the countersink have to be overcome. The magnitude of these moments of friction is dependent on the coefficients of friction which prevail at the surfaces rubbing against one another, with the result that, for the same tightening torque, the pretensioning force is higher in the case of lower coefficients of friction than in the case of higher coefficients of friction.
The magnitude of the bolt pretensioning force and therefore of the pressure between the connecting eye and the wheel hub should, however, be within predetermined limits for a specified tightening torque, since too high a bolt pretensioning force reduces the elasticity of the wheel, with the result that the wheel nuts or wheel bolts can become loose under dynamic load while driving. Too low a bolt pretensioning force can likewise lead to a loosening of the wheel nuts or wheel bolts under dynamic load while driving.
The influences or measures which change the coefficients of friction of the contact surfaces of the wheel nuts or wheel bolts and of the countersink can increase or decrease the bolt pretensioning force and therefore the pressure to a dangerously great extent. The same applies to the influences or measures which change the coefficient of friction of the threads of the wheel nuts or wheel bolts. For example, rust on the surfaces of the wheel nuts or wheel bolts, of the countersink and of the threads, which surfaces are in contact with one another, increases the coefficient of friction.
If the wheel nuts or wheel bolts are provided in an impermissible manner with lubricant, the corresponding coefficients of friction decrease, with the result that a higher bolt pretensioning force is obtained at the same tightening torque.
Coated center disk wheels have, in the countersunk region, a surface protection which as a rule is composed of a pretreatment in the form of zinc phosphating and a primer in the form of a cathodic electrocoating finish. A surface of the countersunk region which is protected in this manner has a lower coefficient of friction than an untreated, bare surface, which may tend to rust.
Wheel nuts and wheel bolts are exposed to attack by moisture on their surfaces which are not in contact with the countersink of the wheel and the threads and are therefore protected from corrosion by a surface coating. Such a surface coating is described, for example, in WO 00/11238. When wheel nuts or wheel bolts protected from corrosion by a surface coating are used for fastening coated center disk wheels, the coefficient of friction at the contact surfaces between the wheel nuts or wheel bolts and the countersink can be reduced to such an extent that the bolt pretensioning forces assume values which are so high that the elasticity of the wheel is reduced to a degree such that the dynamic load when driving can cause the bolts or nuts to become loose.
Moreover, the high pretensioning force results in a very high pressure of the wheel nut or wheel bolt on the paint coat in the countersink. As a result of the heating of the wheel hub region which occurs when driving and the pressure applied to the paint coat in the countersink by the wheel nut or the bolt head, instabilities may occur in the paint coat of the countersink. This results in a further reduction in the coefficient of friction at the contact surfaces between the wheel nuts or the bolt heads and the countersink of the connecting eye in the case of coated center disk wheels. However, if the coefficient of friction decreases in this manner after mounting of the wheel, the wheel bolts or wheel nuts may become loose owing to the dynamic load of the screw union region during driving and owing to the total elasticity of the center disk wheel itself.
The use of wheel bolts protected from corrosion by a surface coating for fastening cast light metal wheels, such as, for example, cast aluminum wheels, does not as a rule result in a reduction in the coefficient of friction at the contact surfaces between the wheel bolt and the countersink, which reduction causes loosening of the wheel bolts, because the surface in the region of the wheel bolt head receptacle in the case of the cast light metal wheel is not covered with a protective layer but is left bare after machining.
For commercial reasons, it is expedient if the same wheel bolts which are used for cast light metal wheels can also be used for coated sheet-metal center disk wheels. The use of wheel bolts of the same design both for cast light metal wheels and for coated sheet-metal center disk wheels also rules out the possibility that, for cast light metal wheels and coated sheet-metal center disk wheels, wheel bolts of different design will be accidentally used or misused for the wheel type for which they are actually inadequate.